Overwhelming evidence exists supporting the benefit of lifestyle and nutritional interventions to prevent or delay type 2 and gestational diabetes and improve glycemic control and co-morbidities in patients of all sub-types of diabetes mellitus. Therefore, nutritional therapy is an indispensable and fundamental treatment component, which has to be based on evidence-based recommendations, adapted for dietary intake and medication, and periodically adapted according to diagnosis and individual course of illness. This overview is based on the currently valid evidence-based nutritional recommendations of the European and American Diabetes Associations for the management of diabetes mellitus. It describes the quality and quantity of beneficial macronutrient (carbohydrates, fat, and protein) and micronutrient intake, alcohol consumption, and food groups. Moreover, the evidence for supplements and functional foods is summarized and the role of body weight and different weight loss diets are discussed.